The present invention relates to an optical communication module that is applied to a wavelength division multiplexing optical communication system. That is, the invention provides a stable optical system for locking a lasing wavelength of light from a laser source and a control system for the optical system. This optical system can be separately operated as a wavelength locker module, but it also can be integrated into an optical communication module having a laser source.
Optical fiber communication features a long transmission length, high speed and large capacity, and a strong immunity to electromagnetic noises; and, hence, a communication system that assures high reliability can be provided. Formerly, in such systems, light of a single wavelength was transmitted on a single strand of optical fiber. However, with the advent of large capacity computerization in recent years, there has been a strong demand for the transmission capacity to be further increased. Therefore, a wavelength division multiplexing optical communication system has been developed and put into practical use, in which a plurality of optical signals each having a different wavelength are transmitted over a single strand of optical fiber, so that the number of communication channels is increased to achieve a system having a larger capacity. Normally, for the wavelength of light to be transmitted in an optical fiber, use is made of wavelength bands where the transmission loss of the optical fiber is low, and such wavelength bands in a 1.3 xcexcm range and in a 1.5 xcexcm range are called windows of transmission. Since the wavelength widths of these windows are limited, the narrower the wavelength spacing between adjacent channels becomes, the more the number of transmission channels can be increased. Presently, the frequency spacing is set to 200 GHz and 100 GHz, but there is a trend toward further narrowing of the frequency spacing, such as to 50 GHz and 25 GHz. Converting the above-mentioned frequency spacings into wavelength spacings, those values become as narrow as approximately 1.6 nm, 0.8 nm, 0.4 nm, and 0.2 nm. When the wavelength spacing is narrowed to such levels, it becomes necessary for the wavelength of the laser source to be controlled to a constant value with pinpoint accuracy. This is because, if the wavelength of the laser source fluctuates to reach as far as the wavelength of the adjacent channel, there occurs crosstalk with the adjacent wavelength channel at the reception side, and, hence, the reliability of information transmission can not be assured. These wavelength (or, frequency) channels are called ITU-T (INTERNATIONAL TELECOMMUNICATION UNION-TELECOMMUNICATION STANDARDIZATION SECTOR) grids and are acknowledged widely as an ITU recommendation.
On the basis of the aforementioned considerations, there have been proposed several methods for controlling the wavelengths of the laser sources for the wavelength division multiplexing of optical communication systems. For example, a method has been devised for locking the wavelength of the laser diode by introducing a dielectric multi-layer filter, a Fabry-Perot etalon, or the like as a wavelength filter and using feedback to control the wavelength on the basis of the operating temperature of the laser diode. Among these wavelength filters, especially the etalon has characteristics such that transmission peaks appear repeatedly in the wavelength according to the number of orders of multi-interference, and therefore, by adjusting the periods of the transmission curve to the ITU-T grids, a single wavelength filter can be used to lock a plurality of wavelength channels. For example, JP-A-79723/1998 discloses a method of locking the wavelength by dividing light which has passed through the etalon into two portions, detecting the two portions using respective photo detectors, and subtracting one signal from the other signal to derive a wavelength deviation signal, which will be used to lock the wavelength.
It is a first object of the present invention to stabilize the wavelength of a laser diode. More specifically, it is an object of the invention to achieve stabilization of a wavelength locking system utilized in a semiconductor laser module (hereinafter referred to as a xe2x80x9claser diode modulexe2x80x9d) in which the wavelength locking optical system is incorporated.
Among the transmitted light beams of the etalon utilized by the wavelength locking system, any light beams that contribute to the wavelength deviation detection effectively are almost collimated light beams. Accordingly, light reflected from the etalon goes back to the laser diode via a converging lens, is reflected by a facet thereof, and is reflected again by the etalon, which is repeated to cause multiple reflections. Therefore, the reflected waves multiply-interfere with one another; and, consequently, a distribution of the interference fringes varies in response to variation of the wavelength. Thus, light that arrives at a photo detector fluctuates, thereby to generate a ripple in the output, and so there arises a problem of instability in this wavelength deviation signal.
It is a second object of the present invention to eliminate or alleviate the external feedback noise in a laser diode. That is, in the operation of a laser diode, there is the problem that so-called external feedback noise (returned light noise) is generated, namely a fluctuation in the lasing intensity resulting from contention of the lasing mode of the laser diode itself with an external resonance mode that is generated by light returned to the laser diode being coupled to a waveguide thereof.
A basic form of the present invention consists of a laser diode module that comprises at least a laser diode device; a first detector element for receiving, directly or indirectly, a first light beam that is obtained when at least one of the light emissions of said laser diode device is divided into two light portions, each traveling in a different direction; a second detector element for receiving a second light beam that comprises the other of the divided light beams at least via a wavelength selective member; and means for controlling the lasing wavelength of the above-mentioned laser diode device on the basis of outputs of the above-mentioned first and second-photo detector elements, wherein a gap between the above-mentioned laser diode device and the above-mentioned wavelength selective member is formed so as to constitute an optical resonator with multi-interference eliminated or alleviated therein.
The following description provides typical examples of the construction of an optical cavity in which multi-interference between the above-mentioned laser diode device and the above-mentioned wavelength selective member is eliminated. The first scheme involves a technique in which the polarization directions of the light emitted from the light source and of light returning to the light source, as returned light, are made different from each other. A more concrete example is as follows. The above-mentioned wavelength selective member in an optical path between the laser diode and the above-mentioned wavelength selective member itself is arranged so as to generate reflected light having a degree of polarization different from that of the incident light falling on the wavelength selective member. In the most preferable form, this difference between the directions of polarization is such that the directions of polarization are mutually orthogonal, which can ensure the most stable operation.
The second scheme involves reduction of the mutual superposition of a light beam emitted from the light source and a light beam that returns to the light source as returned light. For example, the angle of the reflection surface is tilted to avoid a superposition of the incident light and the reflected light. A straightforward configuration for this scheme is as follows. When the incident light is vertical to the surface, the surface is tilted with the respect to the optical axis of the incident light.